Hummingbirds vs. Micro UAVs: Who's the Best Flyer?

Photo: Pslawinski/Wikipedia

Hummingbirds have been optimizing their design for something like 42 million years. Humans have been optimizing our designs for robots that try to fly like hummingbirds for, um, somewhat less time.* So it's not a surprise that hummingbirds are better fliers than robotic microcopters, but it is a surprise that we're actually getting close in efficiency, according to a new study published in the Journal of the Royal Society Interface

The study was done by Stanford University engineers in collaboration with researchers from Wageningen University and Eindhoven University of Technology, in the Netherlands, and the University of British Columbia, in Canada.

Image: Stanford/Journal of the Royal Society Interface
Dried hummingbird wings ranked from highest to lowest aspect ratio (left to right, top to bottom). Species: An, Anthracothorax nigricollis, At, Amazilia tzacatl, As, Amazilia saucerottei, Ca, Calypte anna, Cc, Colibri coruscans, Cm, Chaetocercus mulsant, Dl, Doryfera ludovicae, Ev, Eriocnemis vestita, Fm, Florisuga mellivora, Ha, Haplophaedia aureliae, Ll, Lafresnaya lafresnayi, Mt, Metallura tyrianthina.

I'd love to be able to tell you that the researchers rigged up a bunch of different live hummingbirds with little sensor backpacks and put them in motion capture environments full of high-speed cameras and smoke machines and lasers and stuff to figure out how efficient their wings are. But, no. Instead, they got hummingbird wings (just the wings) from a museum collection, representing 12 different species [pictured above]. They set the wings up to rotate like helicopter blades, and measured lift and drag while visualizing airflow using a specially constructed apparatus [below].

Image: Stanford/Journal of the Royal Society Interface

Comparing state of the art microcopter blades with the hummingbird wings showed that the blades are not bad at all: they're about on par with your average hummingbird, in terms of efficiency. And spinning is much more effective than flapping: if hummingbirds could spin like helicopters do, they'd be twice as efficient as they are now. The very best hummingbird wings, however, were a solid 30 percent more efficient than synthetic blades. Check out this little guy:

This is a substantial difference, and some of it might be related to the aspect ratio (ratio of length to width) of the wings. The best hummingbird wings (belonging to the Anna's hummingbird) have an aspect ratio of 3.9, while the best rotor design had an aspect ratio of 4.7.

There's a lot more going on hummingbird wings versus drone wings, of course. Rotary motion may be the most efficient way to hover, but hummingbirds use their wings for maneuvering as well, and they're incredible at it. If we start to consider how hummingbird-inspired wing design improvements might be applied to flapping wing micro UAVs, we might also see more agility at the same time.

What would be really awesome would be to see whether AeroVironment's Nano Hummingbird might benefit from more direct bioinspiration, although it's pretty freakin' nimble already. And I guess the other way to look at this research is that it suggests that hummingbirds should try and evolve a way to spin rapidly in place while hovering, to maximize their efficiency.

[ Paper ] via [ Stanford ]

* 42 million years ago, for the record, humans were in the process of diverging from squirrel monkeys, and designing flying robots was far less important than, say, bananas.

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